Once you have the first part of the path, use the vl-directory-files function with the with the second part as a pattern to look for and filter for directories only.
Code:
(setq drive "J:\\"
job (getstring "\nSpecify job number (without dashes): ")
prefix1 (substr job 1 3)
prefix2 (substr job 4 4)
project (vl-directory-files
(strcat drive prefix1)
(strcat prefix2 "*.")
-1
)
)
You may have to adjust this code regarding the vl-directory-files function as I haven't tested it. My folder structure is completely different.